Membrane and the use thereof

ABSTRACT

The invention relates to a membrane that consists of a polymer or a polymer mixture. The aim of the invention is to produce membranes, especially planar membranes or hollow fiber membranes that have an as high a separation efficiency as possible while simultaneously having a high membrane efficiency. To this end, the membrane has a gas- or liquid-permeable bicontinuous foam structure with orifices in the interior whose diameters do not exceed 500 nm.

The invention relates to membranes that can be used for separatinggases, for micro- or ultra-filtration and in particular for medicalpurposes, such as for haemodialysis, haemodiafiltration, plasmapheresisor immunotherapy.

German patent DE-A-19 520 188 discloses a process for producingpolymeric hollow fibre membranes in which a molten polymer is used toproduce the hollow fibre membrane using an extrusion apparatus, whereinthe polymer is charged with pressurized gas before the melt enters theextrusion tool of the extrusion apparatus and wherein as a result of apredetermined pressure drop as the polymer exits the extrusion apparatusand the concomitant expansion of the gases in the polymer, a poroushollow fibre membrane is produced. The open porosity and pore sizeproduced thereby does not produce satisfactory separation results, asthe percentage of open porosity is too small and the pores are toolarge. The pore size determines the separation effect and the degree ofopen porosity determines the membrane efficiency.

International patent application WO-A-91/08 243 describes a process forproducing open celled polyurethane foams by mixing a diisocyanate, ahydrogen donor, at least one surfactant, at least one catalyst and ablowing agent, preferably carbon dioxide, pressurizing the mixture in amixing zone in order to keep the blowing agent liquid at ambienttemperature, ejecting the mixture into an environment at atmosphericpressure to evaporate the blowing agent instantaneously, and curing theresulting foam at ambient temperature. This process suffers from thesame disadvantages as those given for the process cited above.

The older but not previously published German patent application No. 19907 824.6 pertains to a membrane that can be produced by forming apolymer or polymer mixture is brought into the desired form, which ischarged with a gas at a pressure that is above atmospheric pressureprior to or after forming, and then by foaming the gas-charged polymerat a temperature that is above the glass transition temperature of thepolymer/gas mixture, and then by stabilizing the foam structure bycooling, in which the gas-charged polymer is foamed with a quantity of afluid that dissolves or swells the polymer. When producing suchmembranes, the solvent residues have to be removed by washing and thesolvent must be recycled for practical reasons.

The aim of the invention is to produce membranes, in particular flat orhollow fibre membranes, with as high as possible a separation effect andsimultaneously with high membrane efficiency.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a membraneis provided. The membrane comprises a polymer or polymer mixture, andthe polymer or polymer mixture has a gas- or fluid-permeablebicontinuous foam structure with internal openings with a diameter ofless than 500 nm. The diameter of the internal openings in the foamstructure may be less than 250 nm or less than 200 nm.

In accordance with another embodiment of the present invention, a methodof forming a membrane is provided. The method comprises forming apolymer or polymer mixture into a desired form, charging the polymer orpolymer mixture with a gas at a pressure that is above atmosphericpressure prior to or after the forming, foaming the gas-charged polymeror polymer mixture at a temperature that is above the glass transitiontemperature of the polymer/gas mixture, and stabilizing the foamstructure to form the membrane. The gas-charged polymer or polymermixture may be foamed at a gas concentration in the polymer or polymermixture that is above a critical concentration and at a temperature thatis lower than a critical temperature. In accordance with yet anotherembodiment of the present invention, a method of using a membrane isprovided. The method comprises providing a membrane comprising a polymeror polymer mixture, wherein said polymer or polymer mixture has a gas-or fluid-permeable bicontinuous foam structure with internal openingswith a diameter of less than 500 nm, and using the membrane for medicalpurposes. The medical purposes may be selected from haemodialysis,haemofiltration, haemodiafiltration, plasmapheresis, immunotherapy,micro- or ultrafiltration, and gas separation.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the preferred embodiments of thepresent invention can be best understood when read in conjunction withthe following drawings in which:

FIG. 1 is a foam diagram for the polyetherimide (Ultem 1000-1000) andcarbon dioxide system of one embodiment of the invention.

FIG. 2 is a scanning electron microscope exposure of a membrane preparedas described in comparative Example 1, not in accordance with theinvention.

FIG. 3 is a scanning electron microscope exposure of a membrane preparedas described in Example 5, in accordance with the invention.

FIG. 4 is a scanning electron microscope exposure of a membrane preparedas described in comparative Example 2, not in accordance with theinvention.

FIG. 5 is a scanning electron microscope exposure of a membrane preparedas described in Example 6, in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides a membrane consisting of polymer or a polymermixture and which is characterized in that it has a gas or fluidpermeable bicontinuous foam structure with internal openings with adiameter of less than 500 nm, preferably less than 250 nm, in particularless than 200 nm.

These membrane properties can be obtained with membranes that areproduced by forming a polymer or polymer mixture into the desired form,charging with a gas at a pressure that is above atmospheric pressureprior to or after forming, and in which the gas-charged polymer orpolymer mixture is then foamed at a temperature that is above the glasstransition temperature of the polymer/gas mixture and then the foamstructure is stabilized, in which the gas-charged polymer or polymermixture is foamed at a gas concentration in the polymer or polymermixture that is above a critical concentration and at a temperature thatis lower than a critical temperature. The foam structure is stabilizedby cooling, preferably by quenching using an ethanol/water mixture.

The membranes of the invention have a percolating, micro- to nano-porousmorphology and thereby exhibit a high membrane efficiency and goodseparation. On foaming, they do not need any polymer-dissolving or-swelling fluid, and so the disadvantages of an additional washing stageand solvent recycle stage are overcome.

Percolation of the membranes can be determined and evaluated as follows:

a) Using Scanning Electron Microscope Exposure:

-   -   The sample is broken in liquid nitrogen and the broken edges are        analyzed. If the scanning electron microscope exposures exhibit        openings or breaks in the foam morphology, this is indicative of        percolation.

b) Using Flow Measurements:

-   -   A sample is fixed in a suitable measurement device, and the ends        of the membranes are then broken in liquid nitrogen. The sample        is then embedded in resin, so that the top and sides are        completely covered. An open end is then fed with a gas or a        liquid which can be pressurized. A gas or liquid is then        admitted onto an open end, which can be carried out under        pressure. Measuring the volume flow of the gas or liquid allows        the percolation to be characterized. Percolation is indicated by        a significantly higher flow compared with closed pore samples.

c) Using Capillary Rise:

-   -   A sample (length: 4 cm, width: 1 cm, thickness: ≦300 mm,        thickness of foamed film) is fixed into an appropriate measuring        apparatus, in which the ends of the polymer sample are broken in        liquid nitrogen. The ends of the fixed sample is immersed for        about 2 to 3 mm in a liquid or solution. After a suitable        period, the rise of the fluid level in the membrane is measured.        The weight change and rise allows the percolation to be        characterized. Closed pore samples do not take up any liquid,        and no rise can be measured.

Three types of processes can be considered for producing the membranesof the invention, which processes are known per se.

The first process is the pressure cell process, which is carried outbatchwise and in which the polymer or polymer mixture is initiallyformed and is then charged with a gas under a pressure that is higherthan atmospheric pressure and at a temperature that is below the glasstransition temperature of the polymer/gas mixture. The temperature isthen raised to a temperature that is above the glass transitiontemperature but below the critical temperature of the polymer/gasmixture, by immersing in a heating bath, and then the gas is driven outof the formed body to produce the desired bicontinuous foam structure.Transfer from the pressure cell to the heating bath must be carried outas fast as possible, so that the dissolved gas can quickly diffuse outof the polymer at ambient pressure. After foaming, the polymeric formedbody must be quenched in an ethanol/water mixture at about 20° C.

The second process type is termed the autoclave process, in which thepolymer or polymer mixture is charged with the gas at a temperature thatis above the glass transition temperature of the polymer/gas mixture andfoaming is induced by spontaneous release of the pressure. In contrastto the pressure cell process, in which the gas-charged polymer isnormally transferred to a heating bath to raise the temperature to abovethe glass transition temperature but below the critical temperature ofthe polymer/gas mixture, the autoclave process does not need a heatingstage as the polymer is already at the required temperature that isabove the glass transition temperature on charging with the gas.

The third process type is termed the extrusion process, in which a meltof the polymer or polymer mixture is charged with the gas in anextrusion tool. Foaming occurs as the melt is ejected from the extrusiontool and the pressure thereby drops. In contrast to the two processesdescribed above, this process is a continuous process.

The pressure cell process is preferred for use in producing themembranes of the invention.

The gas concentration in the polymer or polymer mixture and thetemperature of foaming determine whether a bicontinuous foam structurewith a micro- to nano-porous percolating morphology in accordance withthe invention can be obtained.

The saturation period in the pressure cell depends on the polymer orpolymers used and on the critical concentration to be obtained. Thetransfer period from the pressure cell to the heating bath must, asmentioned, be as short as possible and ensure that on foaming, i.e., inthe heating bath, the concentration of gas in the polymer or polymermixture remains above the critical concentration. Altering the foamingperiod in the heating bath can change the percolation. For reproducibleproduction, it is therefore necessary to keep the transfer period andfoaming period constant by using industrial instrumentation and to keepthe saturation period in the pressure cell constant for a particularsystem of polymers and foaming gas.

The membranes of the invention can be a flat membrane, a hollow fibremembrane or a fibre membrane (monofilament membrane). Preferably, theyare formed from an amorphous or semi-crystalline polymer or polymermixture. Examples of polymers are polyimides, polyetherimides andpolyethersulphones as well as polymer mixtures that contain at least 20%of at least one of said polymers. The polymers can be mixed so thatnormal pressure conditions can readily be employed to obtain asaturation concentration above the critical concentration. The criticalconcentration that on foaming, i.e., in the case of the pressure cellprocess which must be carried out in the heating bath, is suitably atleast 40, preferably at least 43, in particular at least 45, moreparticularly at least 47 cm³ (STP)/cm³ of the polymer or polymermixture. “STP” means standard temperature and pressure, namely 0° C. and0.1013 MPa.

Although the invention is not limited to a special foam or charging gas,CO₂ is preferred.

Foaming is carried out at a temperature of at least 10° C. below thecritical temperature. For each special combination of a polymer orpolymer mixture and charging gas, a foam diagram has to be producedafter determining the glass transition temperature of that polymer/gasmixture and the critical temperature of that polymer/gas mixture, whichdiagram includes, in the zone between the glass transition temperatureand the critical temperature, a region above the critical gasconcentration in which the morphology is bicontinuous. FIG. 1 shows thefoam diagram for the polyetherimide (Ultem 1000-1000)/carbon dioxidesystem. The abscissa shows the concentration dissolved of carbon dioxidein cubic centimetres of CO₂ (STP)/cm³ (polymer). The ordinate shows thefoaming temperature in degrees Celsius. The line labeled 1 describes thecritical temperature of the system, above which a foamed morphology canno longer be obtained. The line labeled 2 describes the glass transitiontemperature of the described system below which a foamed morphologycannot he obtained. This means that a foamed morphology can only beobtained in the zone lying between lines 1 and 2. With carbon dioxideconcentrations of <6 cm³ CO₂ (STP)/cm³ (Ultem 1000-1000), determiningthe critical concentration is experimentally difficult. However, aconcentration of 0 cm³ CO₂ (STP)/cm³ (Ultem 100-1000), can be expectedto have a critical concentration that equals the glass transitiontemperature of the pure polymer. The glass transition temperature of thepure polymer is shown on this diagram as the point where line 2 cuts theordinate at 0 cm³ CO₂ (STP)/cm³ (Ultem 1000-1000). The zone delineatedas 3 shows the zone on the foam diagram in which bicontinuous membranestructures can be obtained. In this case, the foam diagram was onlydetermined up to a carbon dioxide concentration of 57 cm³ CO₂ (STP)/cm³(Ultem 1000-1000). The structures obtained between lines 1 and 2 butoutside zone 3 exhibit a closed morphology.

For each polymer/charging gas system, a unique foaming diagram can beobtained and the position of the zone in which a bicontinuous membranestructure can be obtained can be determined. This zone can be altered ifthe foaming period, polymer properties and transfer time are changed.These parameters, therefore, are preferably kept constant in the processfor producing the membrane of the invention.

The solubility of the charging gas (preferably CO₂) in polymers ofpolymer mixtures for determining which polymers and polymer mixtures canbe used for producing the membranes of the invention can be determinedas follows:

To determine the gas solubility of carbon dioxide in polymers andpolymer blends, an apparatus is used that is similar to that developedby Koros and Paul (Koros, W J, Paul, D R, Design Conditions forMeasurement of Gas Sorption in Polymers by Pressure Decay, J. Polym.Sci., 1976, 14, 1903-1907).

The sorption isotherms are determined at 25° C. up to a pressure of 5MPa for carbon dioxide. The concentration of dissolved carbon dioxide isgiven in cubic centimetres of CO₂ at 0° C. and o.1013 MPa (standardtemperature and pressure) (STP) per cubic centimetre of polymer; cubiccentimetres of CO₂ (STP)/cm³ (polymer).

It is also possible to use other volumetric or gravimetric methods thatallow the solubility of a gas in a polymer to be determined.

The glass transition temperature (Tg) and the critical temperature ofthe polymer/gas mixture and the critical gas concentration aredetermined as follows:

Determination of the Glass Transition Temperature (T_(g)) of Polymer/GasMixtures

Portions of the produced polymer film (polymer or polymer blend) aretreated with the gas or gas mixture in a pressure cell. The saturationtemperature is 25° C. Preferably, saturation is carried out with the gasor gas mixture at the temperature at which the sorption isotherm is alsodetermined. This enables the determined gas pressures (equilibriumpressures) to be readily converted into concentration data. The gaspressure at which saturation with a gas (for example carbon dioxide) iscarried out is termed the saturation pressure. Varying the saturationpressure and the saturation temperature can increase the correspondingsaturation concentration.

The saturation period must be selected so that a homogeneousconcentration profile is obtained. In the case of carbon dioxide as aphysical blowing agent and for amorphous polymer films with a thicknessof more than 100 μm, a saturation period of about 2 h is satisfactory.

After spontaneously releasing the saturation gas (for example carbondioxide) from the pressure cell, the polymer sample is removed andimmersed in a heating bath. Then the foaming stage is carried out. Theimmersion period is about 30 seconds. During immersion, the sample ismoved constantly in the heating bath medium. Care should be taken thatthe polymer sample is completely immersed in the heating bath medium.The heating bath medium must be selected so that the polymer sample isneither physically nor chemically attacked during the foaming stage.After foaming, the plastic sample is quenched in an ethanol/watermixture (about 20° C.), preferably to a temperature below the glasstransition temperature of the polymer.

A plurality of samples saturated at the same pressure with the blowingagent (for example carbon dioxide), are foamed at differenttemperatures. The temperatures at which the polymer sample remainstransparent are determined along with those temperatures at which thepolymer sample becomes opaque and milky. Further limitation of thistemperature zone by foaming the polymer samples at temperatures that liein the first zone allows the transition temperature (glass transitiontemperature) for this saturation pressure or the correspondingsaturation concentration to be determined to within a few degrees. Theoptically determined glass transition temperature can be confirmed usingthe scanning electron microscope (SEM). Samples that are heated to atemperature that is lower than the glass transition temperature of thepolymer/gas mixture exhibit a dense structure. Samples that are heatedto a temperature that is above the glass transition temperature of thepolymer/gas mixture exhibit cell formation. The glass transitiontemperature of a polymer/gas mixture is defined here as the mean betweenthe temperature at which no foaming can be observed and the temperatureat which cell formation can be observed using the TEM.

Varying the saturation pressure means that different blowing agentconcentrations can be used in the polymer. In this way, the glasstransition temperature for different blowing agent concentrations can bedetermined for a polymer/blowing agent mixture using the describedfoaming technique.

Determination of Critical Temperature of Polymer/Gas Mixtures

The critical temperature is defined as the foaming temperature abovewhich no more foamed polymer morphology can be obtained, i.e., abovewhich the density of the sample obtained after foaming is close to thedensity of the polymer sample used (>95% of the starting value). Thecritical temperature is above the glass transition temperature of thepolymer/gas mixture.

Polymers of the polymer film produced (polymer or polymer blend) aretreated with the gas or gas mixture in a pressure cell. The saturationtemperature is 25° C.

The saturation period must be selected so that a homogeneousconcentration profile can be obtained. In the case of carbon dioxide asa physical blowing agent and for amorphous polymer films with athickness of more than 100 μm, a saturation period of about 2 h issatisfactory.

After spontaneously releasing the saturation gas (for example carbondioxide) from the pressure cell, the polymer sample is removed andimmersed in a heating bath. Then the foaming stage is carried out. Theimmersion period is about 30 seconds. During immersion, the sample ismoved constantly in the heating bath medium. Care should be taken thatthe polymer sample is completely immersed in the heating bath medium.The heating bath medium must be selected so that the polymer sample isneither physically nor chemically attacked during the foaming stage.After foaming, the plastic sample is quenched in an ethanol/watermixture (about 20° C.), preferably to a temperature below the glasstransition temperature of the polymer.

-   -   A plurality of samples saturated at the same pressure with the        blowing agent (for example carbon dioxide), are foamed at        increasing temperatures. It is observed that above a set        critical temperature, a foamed polymer morphology can no longer        be obtained. This is demonstrated by the fact that the polymer        film is still (milky) transparent after foaming and the density        of the foamed material is close to the density of the starting        material (density of “pure” polymer). This can be confirmed        using scanning electron microscopy (SEM). Samples heated to a        temperature above the critical temperature display a dense        non-cellular structure, while samples that have been heated to a        temperature below the critical temperature exhibit a cellular        structure.

Varying the saturation pressure allows different blowing agentconcentrations to be employed in the polymer. In this manner, thecritical temperature can be determined for different foaming mediumconcentrations using the foaming techniques described for apolymer/foaming medium mixture.

The critical temperature is dependent, inter alia, on the foaming periodand on the concentration of the dissolved gas (for example the carbondioxide concentration). For shorter foaming periods, the criticaltemperature is higher. Raising the concentration of the dissolved gascan drop the critical temperature. This means that for each foamingperiod and concentration of the dissolved gas, the critical temperaturemust be determined anew. This is particularly the case when a foamingprocess that is different from that described is used, as in this casethe foaming period can vary widely from the values described.

Determination of Critical Gas Concentration

The critical gas concentration is defined as the lowest gasconcentration (preferably the saturation gas concentration) above whichat a foaming temperature limited by the glass transition temperature ofthe polymer/gas mixture as the lower limit and the critical temperatureas the upper limit for the corresponding gas concentration (saturationconcentration), percolation behaviour can be observed using the methodsdescribed at b) or c) below for one or more foamed polymer samples.

-   a) In the Pressure Cell Method    -   Parts of the polymer films produced (polymers or polymer blends)        are treated with the gas or gas mixture in a pressure cell. The        saturation temperature is 25° C.    -   The saturation period must be selected so that a homogeneous        concentration profile is produced. In the case of carbon dioxide        as the physical foaming medium and amorphous polymer films with        a thickness of about 100 μm, a saturation period of about 2 h is        satisfactory.    -   After spontaneously releasing the saturation gas (for example        carbon dioxide) from the pressure cell, the polymer sample is        removed and immersed in a heating bath. Then the foaming stage        is carried out. The immersion period is about 30 seconds. During        immersion, the sample is moved constantly in the heating bath        medium. Care should be taken that the polymer sample is        completely immersed in the heating bath medium. The heating bath        medium must be selected so that the polymer sample is neither        physically nor chemically attacked during the foaming stage.        After foaming, the plastic sample is quenched in an        ethanol/water mixture (about 20° C.), preferably to a        temperature below the glass transition temperature of the        polymer.    -   A plurality of samples, saturated with the blowing agent at the        same pressure (for example carbon dioxide), are foamed at        increasing temperatures. All samples that are foamed above the        glass transition temperature of this polymer/gas mixture and        below the critical temperature of this polymer/gas mixture are        examined for their percolation behaviour.-   b) In the Autoclave Method    -   In the autoclave method described above, saturation with the gas        or gas mixture above is carried out the glass transition        temperature of the polymer/gas mixture, and the foaming stage is        initiated by spontaneously releasing the gas pressure. Thus, the        foaming temperature is identical with the saturation        temperature. If polymer samples are saturated or foamed with the        gas or gas mixture at increasing temperatures, it appears that        above a critical temperature, no more foamed polymer        morphologies can be obtained. By varying the saturation        conditions (gas pressure, temperature), different saturation        concentrations in the polymers or polymer blends can be        obtained. The critical temperature can be determined anew for        these different saturation conditions (saturation        concentrations).-   c) In the Extrusion Method    -   In the extrusion method described above, foam formation occurs        as the polymer/gas mixture exits the extruder head. Varying the        temperature of the extruder head or the temperature of the        extruding polymer/gas mixture enables the temperature above        which no foamed polymer morphology can be obtained to be        determined. As with the autoclave method described above, a        variation in the saturation conditions (gas pressure,        temperature) can produce different saturation concentrations in        the polymer or polymer blend. For these different saturation        conditions (saturation concentrations), the critical temperature        can be determined anew.

To determine the foam diagram of a special polymer/gas mixture, thefollowing is carried out: firstly, the sorption isotherm or isobar isdetermined in order to establish whether the critical concentration canbe achieved. Then the pressure and temperature are adjusted to produce aconcentration of 50 cm³/cm³ of polymer. Then, foaming is carried out atincreasing temperatures, and then the foam characteristics aredetermined, for example by gas flow measurements, to determine thecritical temperature for a particular foaming period.

Flat membranes can be prepared as follows using the pressure cellmethod:

Solutions of the plastic samples (“pure” polymer or polymer blends) areproduced in suitable solvents (tetrahydrofuran, 1-methyl-2-pyrrolidone,dichloroethane, etc.).

Thin films of these polymer solutions are spread onto glass plates usinga spreading knife. The coated glass plates are then dried at about 20°C. to 25° C. in a nitrogen stream in order to evaporate off the majorityof the solvent.

Once the films are “hand dry” and can be removed from the glassplates,—a substantial amount of solvent still remains in the films.Depending on the solvent, plastic and drying conditions, these can bebetween 0.5% and 15% by weight. The films produced are about 100 μmthick.

The films are then dried further in a vacuum drier until the solventconcentration has dropped to below 0.02% by weight.

Extruded films or film parts of the corresponding polymer or polymerblends (free of solvent) can also be used.

The films of the different polymers or polymer mixtures are thensaturated with carbon dioxide at a raised pressure and at ambienttemperature (20° C. to 25° C.) until the gas concentration is over thecritical concentration. The gassing period is about 2 h for films. Thepressures required depend on the polymer used and depend on the sorptioncharacteristics of the corresponding polymer. In each case, a criticalCO₂ concentration must be exceeded. The following concentrations givenfor certain polymers:

Matrimid 5218: 48 ± 5 cm³ CO₂ (STP)/cm³ (polymer) PEI Ultem 1000: 47 ± 5cm³ CO₂ (STP)/cm³ (polymer) PES 7300 P: 47 ± 5 cm³ CO₂ (STP)/cm³(polymer)

These concentrations are concerned with the pressure cell method, and toa saturation period of 120 min at 25° C., and also to a foaming periodof 30 s. A variation in the process means that the critical thresholdconcentration has to be determined anew.

After releasing the pressure and opening the pressure cell, the polymersample is removed and immersed in a heating bath for about 30 seconds.The foaming process then takes place. The temperature of the heatingbath must be above the glass transition temperature of the polymer/gasmixture and below a critical temperature. Then the polymer sample isquenched in an ethanol/water mixture (about 20° C.), preferably at atemperature that is below the glass transition temperature of thepolymer.

EXAMPLE 1

Foaming of a polyimide with structural formula:

10 g of the polyimide Matrimid® 5218 (Manufacturer: Ciba SpecialityChemicals, Performance Polymers, Basel, Switzerland) was dissolved in 40g of tetrahydrofuran (THF) and a 0.50 mm thick film was spread onto aglass plate. The film was dried in a stream of nitrogen (about 20° C.)and then at 30° C. or 150° C. in a vacuum drier until the solventconcentration (THF) was <0.02% by weight. Portions of the film produced(thickness ≦100 im) were saturated at 10, 20 and 50 bars and at ambienttemperature (20° C. to 25° C.) with carbon dioxide in a pressure cellfor 120 minutes. The carbon dioxide-saturated films were foamed attemperatures between 200° C. and 320° C. for 30 s. It was observed thatthe samples that had been saturated with a CO₂ gas pressure of 20 and 50bars with a foaming temperature between 210° C. and 270° C. exhibitedpercolating, bicontinuous structures. Samples that had been saturatedwith a gas pressure of 10 bars of carbon dioxide exhibited nopercolating structure. Above 320° C., for a foaming period of 30 s and asaturating pressure of 50 bars, no foamed morphology could be obtained.Percolation was demonstrated by means of flow measurements.

Foaming conditions:

saturation pressure: 5 MPa saturation gas: carbon dioxide saturationperiod: 2 h foaming temperature: 260° C. foaming period: 30 s

Interpretation of a scanning electron microscope exposure(magnification: 50 000): bicontinuous, percolating morphology withopenings with a size of about 20 to 40 nm. Percolation behaviour wasconfirmed using the described flow measurements.

saturation pressure: 5 MPa saturation gas: carbon dioxide saturationperiod: 2 h foaming temperature: 270° C. foaming period: 30 s

Interpretation of a scanning electron microscope exposure(magnification: 50 000): bicontinuous, percolating morphology withopenings with a size of about 30 to 90 nm. Percolation behaviour wasconfirmed as above.

EXAMPLE 2

Foaming of a polyimide with structural formula:

10 g of the polyimide P84® (Manufacturer: HP Polymer GmbH, Lenzing,Austria) was dissolved in 40 g of 1-methyl-2-pyrrolidone (NMP) and a0.50 mm thick film was spread onto a glass plate. The film was dried ina stream of nitrogen (about 20° C.) and then at 30° C. or 150° C. in avacuum drier until the solvent concentration (NMP) was <0.02% by weight.Portions of the film produced (thickness ≦100 μm) were saturated at 30,40 and 50 bars and at ambient temperature (20° C. to 25° C.) with carbondioxide in a pressure cell for 120 minutes. The carbon dioxide-saturatedfilms were foamed at temperatures between 180° C. and 280° C. for 30 s.It was observed that foaming the carbon dioxide-saturated films attemperatures between 220° C. and 270° C. produced bicontinuousstructures. Percolation was confirmed by means of capillary rise.

Foaming conditions:

saturation pressure: 5 MPa saturation gas: carbon dioxide saturationperiod: 2 h foaming temperature: 260° C. foaming period: 30 s

Interpretation of a scanning electron microscope exposure(magnification: 50 000): bicontinuous, percolating morphology withopenings with a size of about 30 to 150 nm. Percolating behaviour wasconfirmed using capillary rise measurements.

EXAMPLE 3

Foaming of a polyetherimide with structural formula:

10 g of the polyetherimide Ultem® 1000 (Manufacturer: General Electric,Huntersville, N.C., USA) was dissolved in 40 g of chloroform and a 0.50mm thick film was spread onto a glass plate. The film was dried in astream of nitrogen (about 20° C.) and then at 30° C. or 150° C. in avacuum drier until the solvent concentration (chloroform) was <0.02% byweight. Portions of the film produced (thickness ≦100 μm) were saturatedat 50 bars and at ambient temperature (20° C. to 25° C.) with carbondioxide in a pressure cell for 120 minutes. The carbon dioxide-saturatedfilms were foamed at temperatures between 110° C. and 250° C. for 30 s.It was observed that foaming the carbon dioxide-saturated films attemperatures between 170° C. and 205° C. produced bicontinuousstructures. Above 250° C., no foamed morphology could be obtained with afoaming period of 30 s and saturation pressures between 10 and 54 bars.Percolation was confirmed as described in Example 1. A bicontinuous,percolating morphology was obtained.

EXAMPLE 4

Foaming of a polyetherimide with structural formula:

For the polyetherimide Ultem® 1000-1000 (Manufacturer: General Electric,Bergen op Zoom, Netherlands), extruded films with a thickness of 75 imwere used. The film was dried for 24 h at 150° C. in a vacuum drier. Aportion of the treated film was saturated at 30, 40, 46 and 50 bars andat ambient temperature (20° C. to 25° C.) with carbon dioxide in apressure cell for 120 minutes. The carbon dioxide-saturated films werefoamed at temperatures between 130° C. and 250° C. for 30 s. It wasobserved that foaming temperatures between 150° C. and 210° C. couldproduce bicontinuous structures, in which the gas flow was dependent onthe saturation concentration of dissolved CO₂. Above 250° C., for afoaming period of 30 s and saturated pressures of 10 and 54 bars, nofoamed morphology could be obtained. For samples saturated with 30 barsof carbon dioxide, no percolating structure was observed, but rather aclosed cell structure. Percolation was confirmed by means of the flowmeasurements described above.

Foaming conditions:

saturation pressure: 5 MPa saturation gas: carbon dioxide saturationperiod: 2 h foaming temperature: 180° C. foaming period: 30 s

Interpretation of a scanning electron microscope exposure(magnification: 30 000): bicontinuous, percolating morphology withopenings with a size of about 50 to 150 nm. Percolation behaviour wasconfirmed using the flow measurements described above.

Foaming conditions:

saturation pressure: 5 MPa saturation gas: carbon dioxide saturationperiod: 2 h foaming temperature: 195° C. foaming period: 30 s

Interpretation of a scanning electron microscope exposure(magnification: 50 000): bicontinuous, percolating morphology withopenings with a size of about 30 to 70 nm. Percolation behaviour wasconfirmed by means of the flow measurements described above.

COMPARATIVE EXAMPLE 1 AND EXAMPLE 5

Foaming a polyethersulphone with structural formula:

10 g of the polyethersulphone Sumicaexcel® 7300 P (Manufacturer:Sumitomo Chemicals, Japan) was dissolved in 40 g of1-methyl-2-pyrrolidone (NMP) and a 0.50 mm thick film was spread onto aglass plate. The film was dried at 70° C. in a circulated air drier for2 h and then the temperature was slowly raised to 180° C. The dryingperiod at 180° C. was adjusted so that the solvent concentration (NMP)was <0.02% by weight.

A portion of the produced film (thickness ≦100 im) was saturated withcarbon dioxide at:

-   i) 57 bars and 0° C.;-   ii) 50 bars and 20° C.;-   iii) 40 bars and 20° C.;    in a pressure cell for 120 min. The carbon dioxide-saturated film    was foamed at temperatures T_(foam)) between 165° C. and 230° C. for    30 s. Above 230° C., no foamed morphology could be obtained with a    foaming period of 30 s and saturation pressures of 40 and 50 bars.    It was observed that the following combinations (foaming    temperatures and saturation conditions), see Table 1, could produce    percolating, bicontinuous structures. Percolation was confirmed    using the flow measurements described above.

TABLE 1 Production of bicontinuous, percolating Sumicaexcel ® (7300 P)morphologies saturation T_(foam): T_(foam): T_(foam): T_(foam):T_(foam): conditions 165° C. 175° C. 185° C. 195° C. 205° C. 57 bars CO₂percolating percolating percolating percolating percolating  0° C. 50bars CO₂ not not percolating percolating percolating 20° C. percolatingpercolating 40 bars CO₂ not not not not not 20° C. percolatingpercolating percolating percolating percolating

COMPARATIVE EXAMPLE 1

Foaming conditions:

saturation pressure: 4 MPa saturation gas: carbon dioxide saturationperiod: 2 h foaming temperature: 185° C. foaming period: 30 s

Details regarding scanning electron microscope exposure: FIG. 2

Magnification: 20 000

the white mark on the lower edge corresponds to 1 μm.

Interpretation: closed cell morphology with cells of the order of 300 to800 nm, as the critical concentration on saturation with 4 MPa CO₂ wasnot reached.

EXAMPLE 5

Foaming conditions:

saturation pressure: 5 MPa saturation gas: carbon dioxide saturationperiod: 2 h foaming temperature: 185° C. foaming period: 30 s

Details regarding scanning electron microscope exposure: FIG. 3

Magnification: 20 000

The white mark on the lower edge corresponds to 1 μm.

Interpretation: bicontinuous, percolating morphology with openings ofthe order of 200 to 300 nm, as the critical concentration on saturationwith 5 MPa CO₂ was exceeded.

COMPARATIVE EXAMPLE 2

Foaming a polysulphone with structural formula:

10 g of the polysulphone Udel® P-3500 (Manufacturer: BP-Amoco, Belgium)was dissolved in 40 g of tetrahydrofuran (THF) and a 0.50 mm thick filmwas spread onto a glass plate. The film was dried at 20° C. in a streamof nitrogen and then at 30° C. or 150° C. in a vacuum drier until thesolvent concentration (THF) was <0.02% by weight.

A portion of the produced film (thickness ≦100 im) was saturated at 50bars and 20° C. with carbon dioxide in a pressure cell for 120 min. Thecarbon dioxide-saturated film was foamed at temperatures (T_(foam))between 110° C. and 230° C. for 30 s. Above 230° C., no foamedmorphology could be obtained with a foaming period of 30 s and asaturation pressure of 50 bars. It was observed that none of the foamingtemperatures used could produce percolating, bicontinuous structures.Percolating behaviour was confirmed using the flow measurementsdescribed above.

Foaming conditions:

saturation pressure: 5 MPa saturation gas: carbon dioxide saturationperiod: 2 h foaming temperature: 145° C. foaming period: 30 s

Details regarding scanning electron microscope exposure: FIG. 4

Magnification: 5000

The white mark on the lower edge corresponds to 5 μm.

Interpretation: closed cell morphology with cells of the order of 1 μm,as the critical concentration on saturation with 5 MPa CO₂ at 20° C. wasnot reached.

EXAMPLE 6

Foaming a polymer blend consisting of 80% by weight of Udel® P-3500(polysulphone) and 20% by weight of Matrimid® 5218.

8 g of the Udel® P-3500 and 2 g of Matrimid® 5218 were dissolved in 40 gof dichloromethane and a 0.50 mm thick film was spread onto a glassplate. The film was dried at 50° C. in a circulated air drier for 2 hand then the temperature was slowly raised to 195° C. The drying periodat 195° C. was adjusted so that the solvent concentration(dichloromethane) was <0.02% by weight.

A portion of the produced film (thickness ≦100 μm) was saturated at 50bars and at 20° C. with carbon dioxide in a pressure cell for 120 min.The carbon dioxide-saturated film was foamed at temperatures (T_(foam))between 120° C. and 280° C. for 30 s. Above 280° C., no foamedmorphology could be obtained with a foaming period of 30 s and asaturation pressure of 50 bars. It was observed that foamingtemperatures between 170° C. and 200° C. produced percolating,bicontinuous structures were produced. Percolating behaviour wasconfirmed using the flow measurements described above.

Foaming conditions:

saturation pressure: 5 MPa saturation gas: carbon dioxide saturationperiod: 2 h foaming temperature: 190° C. foaming period: 30 s

Details regarding scanning electron microscope exposure: FIG. 5.

Magnification: 20 000

The white mark on the lower edge corresponds to 1 μm.

Interpretation: bicontinuous, percolating morphology with openings ofthe order of 200 to 400 nm, as the critical concentration was exceededon saturation.

1. A method of forming a bicontinuous membrane comprising: forming apolymer or a polymer mixture into a desired form; charging the polymeror polymer mixture with a gas at a pressure that is above atmosphericpressure; foaming the gas-charged polymer or polymer mixture at atemperature that is above the glass transition temperature of thepolymer/gas mixture and below the glass transition temperature of thepure polymer, wherein the gas-charged polymer or polymer mixture isfoamed at a gas concentration in the polymer or polymer mixture that isabove a critical concentration of at least 47 cm 3 (STP)/cm 3 of thepolymer or polymer mixture and at a temperature that is lower than acritical temperature; and wherein the gas-charged polymer or polymermixture is foamed without the addition of a polymer dissolving fluid;and stabilizing the foam structure to form the membrane.
 2. The methodas claimed in claim 1, wherein the temperature is at least 10° below thecritical temperature.
 3. The method as claimed in claim 1 wherein thepolymer or polymer mixture comprises an amorphous or semi-crystallinepolymer or polymer mixture.
 4. The method as claimed in claim 1 wherein,the polymer or polymer mixture is charged with the gas at a temperaturethat is less than the glass transition temperature of the polymer/gasmixture and the temperature is subsequently raised above the glasstransition temperature of the polymer/gas mixture but below the criticaltemperature of the polymer/gas mixture.
 5. The method as claimed inclaim 1 wherein, the gas is charged at a temperature that is higher thanthe glass transition temperature of the polymer/gas mixture but lessthan the critical temperature of the polymer/gas mixture, and whereinthe polymer or polymer mixture is subsequently foamed by pressurerelease.
 6. The method as claimed in claim 1 wherein, a melt of polymeror polymer mixture is charged with the gas in an extrusion tool and isfoamed, by exit pressure release on exiting the extruder, at atemperature that is above the glass transition temperature of thepolymer/gas mixture but less than the critical temperature.
 7. Themethod as claimed in claim 1, wherein carbon dioxide is used as acharging gas.
 8. The method as claimed in claim 1, wherein said step ofstabilizing said foam structure comprises quenching.
 9. The method asclaimed in claim 8, wherein said quenching utilizes an ethanol/watermixture.
 10. The method as claimed in claim 1 wherein said membranecomprises a polyimide, polyetherimide or polyethersulphone polymer. 11.The method as claimed in claim 10 wherein said membrane comprises apolymer mixture containing at least 20% of at least one of thepolyimide, polyetherimide or polyethersulphone polymers.
 12. The methodas claimed in claim 10 wherein said membrane comprises a membrane in theform of a flat or hollow fiber membrane.
 13. The method of claim 1wherein the membrane has a structure with internal openings with adiameter of less than 500 nm, and is capable of being used in medicalapplications.
 14. The method as claimed in claim 13 wherein the medicalapplications are selected from haemodialysis, haemofiltration,haemodiafiltration, plasmapheresis, immunotherapy, micro- orultrafiltration, and gas separation.